New girl is literally an NTR antagonist. People seem to only think that applies to a woman being big-dicked away from the boyfriend or husband but a woman seducing a guy into an affair and then prying him away from the wife or girlfriend is exactly NTR.
It seems everyone's definition is different, but to me NTR is when someone uses sex with the intent to steal someone's significant other. (Unrequited love is a different category which I think is called "but I loved her first".) It could be because they want the love interest themselves or (stronger) they want to hurt the cuckold/cuckquean. (While looking that word up at Wikipedia I found that the female equivalent of the "bull" is called a "cuckcake".) The wife going out on her own and hooking up is just cheating, even if she leaves her cuckold husband, unless she happens to hook up with someone who then intentionally sexes her away.